toolchain: add 4.18.x choice for headers

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 7f62d812)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>